I have spent considerable time pouring over the reviews & comments regarding these 2 lenses. Clearly they both have their respective strengths & weaknesses. And I have concluded that the "best" lens depends upon each person's preference ordering.

However, I am having trouble comparing the image quality of these 2 fine lenses. The reviews/comments on their respective optics & resulting IQ seem to distill down to a virtual tie. Not being a guy who makes lens purchases by rolling the dice, I was hoping to solicit some thoughtful opinions from those who might be familiar with both.

BTW, the lens will be used on a Canon 60D primarily for walk-around travel photography.

Owned both – the CA on the Canon 15-85 f/3.5-5.6 was a definite deal breaker for me, but mainly for landscapes. I've been quite happy with the Canon 24-105 f/4L but on the 60D you are going to miss out on the wide angle side. If you are doing travel photography you will will be upset if you don't have a 28mm or smaller FF equivalent lens.

On the 60D, the 24-105 is not particularly wide -- around 40mm equivalent if my math brain is working this morning. For a walk around lens, I would not be happy with that. On a full frame body, the 24-105 is the first lens I grab when going out for a photo walk. Love it.

The 15-85 is an EF-S lens, so of course it only fits the crop sensor cameras. I've handled one briefly and I was impressed with the build quality and the range (basically the same as the 24-105 on FF.) Given that most lens issues can be fixed in software these days, I'd want that lens for my travel photography on a 60D.

Of course, you may prefer a much longer lens and the 24-105 will still be quite good if you decide to buy a 6D or something similar.

Owned both – the CA on the Canon 15-85 f/3.5-5.6 was a definite deal breaker for me, but mainly for landscapes. I've been quite happy with the Canon 24-105 f/4L but on the 60D you are going to miss out on the wide angle side. If you are doing travel photography you will will be upset if you don't have a 28mm or smaller FF equivalent lens.

Cheers,

Tom: I am not particularly concerned about sacrificing some wide angle of the Canon 15-85 on my 60D since I generally carry a Canon 10-22 as well. I find that this lens serves me well for interior & architectural shots.

But, regarding the CA on the Canon 15-85 ..... did you find this correctable in post-production?

I've owned both these lenses and I think they're both good. If the FL range of one is clearly better for you than the other, then get that one. But I think IQ is clearly (if only slightly) better on the 24-105. Not necessarily sharper, but "smoother" and more contrasty. Also having f/4 at the long end is a definite plus. Build-quality wise, the 15-85 is a lot better than the bottom tier of Canon lenses, but nowhere close to the 24-105. Mine had a bit of zoom creep, with a couple of "bumps" where it wouldn't stay in place unless you held it there. CA was only a minor problem (shooting raw with Lightroom). Vignetting was an annoyance, easily correctable, but I had to be careful with filters - no stacking. With the 24-105 (on crop frame) all this was a non-issue. 24-105 on crop-frame was a great walkaround FL range for me. I shoot Nikon now (and I'd have to say that the Nikon 16-85 is the lens the Canon 15-85 ought to be) but if I could afford full-frame, I'd gladly go back to Canon and those sweet f/4L zooms.

I have a 60D and bought the 15-85mm as my "kit" lens and walk-around lens. If you rarely shoot at a focal length shorter than 15mm, I would say that this is a good one-lens solution. I am very happy with it, but I am not making huge images. I tend to use it at 15mm a lot. I don't know how it does with a 1.4x teleconverter.

Of course there are multiple ways to cover, and if you want the 2-lens solution, I would go for the 24-105mm f/4, since you already have the short end covered.

i've had a lot of use from the 24-105 FF and very apsc. i wish it were better (particularly wide open and at the long end) and there is definitely sample variation resulting from manufacturing tolerances and complexity of the lens (Canon service made a slight improvement in mine, but i suspect that improving performance at one focal length may result in loss at another) - so don't expect one side of the image to be as sharp as the other

but performance is generally good enough for most purposes and the combination of FL range and IS make it really useful (particularly on FF, but paired with the 10-22 will be a good apsc combination). if the new 24-70 proves to be better, i'll probably buy one - but the first test by lensrentals.com are a bit disappointing

Thank you, everyone, for your thoughts & recommendations. For me, this has been a reaffirmation of everything I have read regarding these lenses. The difference in IQ appears to be relatively minor for most users. I am not suggesting that such a difference is unimportant; but merely that (for me) my choice of lens will likely be deferred to other criteria.

On a side note, one of the things that I have learned as my skills as a photographer have improved is that negligible differences in camera/lens quality can be more that compensated for by investing in the cultivation of basic photographic skills. Reminds me of my days in vintage motocross racing. In a quest to increase speed, a fellow competitor mentioned that he had replaced his bike's exhaust pipe to save about 3 lbs. of overall weight. However, since his guy tipped the scales at about 300 lbs. butt-naked, overall track performance would have been more dramatic if he simply went on a modest diet. But, I digress....